4.6.2. Oxidative Stability of Mackerel Fillets

Peroxide and TBARS values were used to determine the oxidative stability of mackerel fillets.

1. Determination of peroxide value

By adopting the procedure described by Shon and Chin [70], the peroxide value of mackerel fillets was calculated. Five grams of fillet sample was heated in a water bath at 60 ◦C for 3 min, followed by the addition of 30 mL of a solution of acetic acid-chloroform (3:2 *v*/*v*) accompanied by thorough mixing by stirring to ensure homogeneity of the sample and also to dissolve the fat. Then a filtration process was carried out, followed by the addition of 0.5 mL of saturated potassium iodide solution to the filtrate. Titration was carried out with a standard solution of sodium thiosulfate (25 g/L) in the presence of a starch solution as an indicator. The value of peroxide was expressed in peroxide equivalent units, in milliequivalent peroxides per kilogram of lipid, which was calculated by the following equation:

$$POV\left(\text{meq.kg}^{-1}\right) = \frac{S \times N}{W} \times 1000\tag{1}$$

where *S* is the volume of titration (mL), *N* is the normality of the sodium thiosulfate solution, and *W* is sample weight (kg).

2. Determination of TBARS (Thiobarbituric Acid-Reactive Substances)

Each fish fillet sample (5 g) was dispersed in 20 mL of thiobarbituric acid solution (0.375% thiobarbituric acid, 15% trichloroacetic acid, and 0.25 mol/L HCl). The mixture was heated in boiling water for 10 min, cooled with water, and centrifuged at 3600× *g* for 20 min at room temperature. Then, the TBARS value of the coated fish fillets was assessed spectrophotometrically at 531 nm by spectrophotometer (Model UV-VIS- 2802PC, USA) according to the method described by Song et al. [49]. TBARS values were expressed in milligrams of malondialdehyde (MDA) per kilogram of fish fillet.
